Apparent sound source translator



Feb. 22, 1966 Filed Nov. 19. 1962 3 Sheets-Sheet 1 B. s. A 74L "EA/TOPSM. R. SCHROEDER LZITTORNEV United States Patent APPARENT SOUND SOURCETRANSLATOR Bishnu S. Atal, Murray Hill, and Manfred R. Schroeder,

Gillette, N .J assignors to Bell Telephone Laboratories,

Iyncolsporated, New York, N.Y., a corporation of New Filed Nov. 19,1962, Ser. No. 238,539 9 Claims. (Cl. 179-1) This invention relates tothe reproduction of sound, and in particular to the method of andapparatus for the production of arbitrarily located sound images withonly two loudspeakers. It has for its principal object the precisecontrol of the direction of the origin of a socalled phantom or virtualsound image in a system in which signals are radiated from two discretepoints only. It is another object of the invention to simulate theeffect of a number of discrete sound sources placed in differentpositions by the appropriate energization of two sound sources in fixedpositions.

With the advent of high quality stereophonic recording and reproduction,much concern has been given to the exploitation of the spatial illusionswhich can be created with only two separate loud-speakers supplied withcorrelated signals. By suitable control of the amplitude and phase ofthe correlated signals the apparent origin of a sound can be placedarbitrarily at either of the two loudspeakers or at any point betweenthem. Some widening of the sound image stage may be achieved bysupplying out-of-phase correlated signals to either or both of tworadiating loudspeakers. This has the effect of pushing the apparentsource further away from the center line of the speaker system. However,in doing this, various undesirable effects are often produced, e.g.,various in the head localizations are created. While these are notnecessarily objectionable, they nevertheless are very often artificialsounding. Another way of broadening the stage involves the use of agreat number of loudspeakers individually supplied with signalsdeveloped by a corresponding number of spaced microphones. Techniques ofthis sort yield excellent results, of course, but are economicallyunattractive because of the great number of individual sound channelsand transducer elements required.

This invention attacks the problem of sound localization in a differentway. A mathematical analysis of the sound reaching each ear from anyarbitrarily placed source is utilized to develop an appropriate filtercharacteristic for each of two channels. Networks with the specifiedfilter characteristic, placed in series with each of two separateloudspeakers, transform the signal radiated by each to one which willproduce at both ears of a listener situated at or near the center linebetween the speakers, a signal identical to one that would be receivedby him from the arbitrarily placed source. In effect, the signalsradiated from the two loudspeakers combine at the listeners ears toproduce a pressure wave corresponding to a signal from an arbitrarilylocated source. To a first approximation, frequency independent networkelements create a reasonably sharp virtual sound image. Frequencysensitive networks, however, have been found to produce very sharplydefined images in any direction in the sound plane.

The two signals required for the creation of one or more arbitrarilylocated sound images, in accordance with the invention, contain all ofthe information on which the listener bases his estimate of the locationof the sound. This includes the relative (frequency dependent) amplitudeof signals emanating from both loudspeakers, and the times of arrival ofthe sound pressure waves at each of his ears. The required filtercharacteristic may be imparted to the two correlated signals at anypoint in a record or reproduce cycle. By use of the apparatus of thepresent invention at the reproducer station to filter signals beforethey are delivered to two loudspeakers, ordinary sterephonic signals maybe given a wider stage width. Alternatively, the filtering may takeplace at a recording station, to create a pair of recorded signals thatcontain the required directional information. Independent playback ofsuch a pair of signals in a twochannel system will consequently yieldthe desired illusion.

The invention will be fully apprehended from the following detaileddescriptions of illustrative embodiments thereof taken in connectionwith the appended drawings in which:

FIG. 1 is a pictorial diagram illustrating the relation to one anotherof sound pressure waves radiated from two loudspeakers in producing anarbitrarily located sound image in accordance with the invention;

FIG. 2 is a block schematic diagram illustrating localization networkapparatus suitable for use in the practice of the invention;

FIG. 3 illustrates suitable frequency characteristics for the frequencydependent amplifiers of FIG. 2 in accordance with the invention;

FIG. 4 is a block schematic diagram showing a multiple-value delayelement suitable for use in the apparatus of FIG. 2;

FIG. 5 is a block schematic diagram of apparatus for processing a pairof input signals in accordance with one mode of operation of theapparatus of the invention; and

FIG. 6 is a block schematic diagram of apparatus for processing aplurality of input signals in accordance with another mode of operationof the invention.

Before entering upon a detailed description of the apparatus of theinvention and of the fashion in which it operates, it is desirable todiscuss certain psychoacoustic (psychological-acoustic) concepts andcertain mathematical relations, some of which are implemented by theapparatus shown in the drawings.

The difference in the amplitude and time of arrival of the transients ofa sound at a listeners two ears provides most of the information uponwhich an auditory judgment of the direction of the origin of the soundis based. Since this invention is concerned primarily with localization,the

pulse is the best stimulus for analysis. Since the short pulseencompasses all frequencies, it stands as an idealization of transientsin all sounds, speech, music, noise, and the like.

The principles of the invention may thus be best described on the basisof single pulses and the manner by which they are used by a listener indetermining the origin of a sound. Consider, for example, thearrangement depicted in FIG. 1. A listener 10 faces two loudspeakers 1and 2 located at equal distances from him and at an angle :0 from thecenter line of his position. If a signal, e.g., a short pulse x (t), isradiated from the left speaker I, the sound pressures at the listenersleft and right cars will be h "(t) and h "(t), respectively. Soundpressure wave h (t) will be the stronger of the two and will reach thelisteners left ear before sound pressure Wave h "(t) reaches the rightear. Consequently, the listener will have no difficulty in correctlylocating the source of pulse x (t) as originating in loudspeaker 1.Similarly, if a signal, e.g., a short pulse, x (t), is radiated fromright speaker 2, the sound pressures at the listeners left and rightears will be h "(t), and h t), respectively. In this case sound pressurewave h (t) will reach the listeners right ear with greater intensitythan the sound pressure wave h "(t) will reach his left ear. Moreover,it will reach his right ear before it reaches his left ear. Again nodiificulty is encountered in correctly locating the origin of the soundas loudspeaker 2. Clearly then, the listener would imagine a pulse tooriginate at phantom loudspeaker 3 positioned at an angle go from thecenter line if the sound pressure wave perceived at his right ear, e.g.,h "(t), reached his right car at a time, depending on the angle (p,before a wave h (t) of a somewhat lesser amplitude reached his left ear.This is precisely what his ear would hear if a sound actually originatedat point 3.

It is in accordance with the present invention to provide at thelisteners left and right ears, the appropriate sound pressure waveswhich would reach his ears from such a source of sound, 3, from the twofixed position loudspeakers 1 and 2.

To create the illusion discussed above, e.g., a pulse originating fromloudspeaker 3, a single pulse is initially radiated from loudspeaker 2.It reaches the listeners right ear at a time which may arbitrarily bedesignated zero time and with a magnitude g=1. At a time T later, thepulse reaches his left ear with a somewhat smaller magnitude g The delay1-,, is a function of the angular position of loudspeaker 2, namely theangle 0. So far as the right ear is concerned, the pulse could Well haveoriginated in loudspeaker 3. However, the attenuated and delayed pulsewhich reaches the left ear destroys this illusion and positivelyestablishes the source as speaker 2. Consequently, the pulse reachingthe listeners left ear is canceled by a pulse radiated from speaker 1.Such a pulse must obviously reach the listeners left ear with amagnitude of inverse polarity to the magnitude of the pulse reaching hisleft ear from speaker 2, i.e., g and at a time T seconds later than theinitial. pulse from speaker 2 reached his right ear. The illusion is notyet complete since the listener expects in his left car, a delayed andattenuated pulse from phantom loudspeaker 3. This pulse also is suppliedby loudspeaker 1 at a time 'r seconds following the reference pulse.With this pulse, the necessary conditions for the establishment of anapparent source at loudspeaker 3 is complete. However, both the pulsefrom speaker I used to cancel the premature pulse from speaker 2, andthe substitute pulse from speaker 1, reach the listeners right ear,Consequently, speaker 2 must provide the proper signals for cancelingthese two pulses at the right car. It will be realized that themagnitudes of these pulses are diminished and that they occur 1-,,seconds following the time of their radiation from loudspeaker 1. Inlike manner, the pulses from loudspeaker 2, used to cancel the unwantedpulses at the listeners right ear, are received T seconds later indiminished form at his left ear. Once again, loudspeaker 1 provides thenecessary canceling signals. Each repetition has an amplitude g timesthat of the previous pulse and is delayed by a time 2T9.

If the above process is continued for a suificiently long time, all ofthe unwanted pulses are eventually canceled out by virtue of theattenuation involved in each subsequent cancellation. All that remains,therefore, at the listeners left and right ears are pulses spaced apartin time and related in amplitude as though they originated in phantomsource 3.

It is thus in accordance with the present invention to energizeloudspeakers 1 and 2 with signals which, when combined at the listenersears, produce pressure waves corresponding to those from any desireddirection. Simply expressed, the signals supplied to loudspeakers 1 and2 are adjusted by means of a suitable localization network in bothintensity and phase in conformity with a prescribed schedule.

From the above discussion it may be observed that if x (t) and x (t) arethe inputs required to produce desired pressure responses y (t) and y(t) at the left and right ears, respectively,

where signifies convolution. If the Fourier transforms are taken forboth sides of Equations 1 and 2 the following relations are obtained:

Where 10 20 1( 2( 1( and z( are the Fourier transforms of x (t), x (t),y (t), y (t), h "(t) and h (t), respectively. If Equations 3 and 4 arethen solved for X (w) and X (w), x (t), and x (t) are obtained by takingthe inverse Fourier transforms:

Providing that a(t) is a realizable impulse rseponse, x (t) and x 0),the required loudspeaker input signals, may be created by implementingEquations 5 and 6. For example, if it is desired to simulate a phantomsound source S at a direction (p from the center line of the listener,it is necessary only that the pressure responses 3 (2) and y (t)obtained by radiating a short pulse from the source S be appropriatelyspecified. For simplicity, it may be assumed that the interaural delayand loss due to the sound source in any direction depend only on theangle and not on the frequency of the radiated sound. The pressureresponses at the left and right ears of the listener may then bespecified as:

where g,,, 7' and g, and 1-,, are the differential gains and delaysproduced at the two ears by sound incident from directions 0 and (,0,respectively.

Substituting for h (t), h (t), y (t) and y (t) in Equations 5 and 6 therequired loudspeaker signals are obtained as follows:

Thus the signals x (t) and x (t) for the left and right loudspeakers 1and 2, respectively, are defined such that the sound pressures at theleft and right ears of the listener will be identical to those whichwould be developed at his left and right cars from a single sourceradiated from loudspeaker S. Merely by altering the parameters of thesignal, the apparent direction of the sound source S may be variedwithin a half plane, that is, anywhere within approximately of eitherside of the center line between speakers 1 and 2.

The required signals x (t) and x (t) may be produced with the apparatusdepicted in FIG. 2. Apparatus of this sort is termed a localizationnetwork. Considering the analysis with a single energizing pulse onceagain, a single pulse applied to input terminal 20 is passed by way ofadder 21 directly to the positive input of subtractor 22 to form theleading pulse of output signal x (t). It thus represents the referencepulse which, in the example given above in connection with FIG. 1,denotes the first pulse received from phantom source 3 at positiondegrees to the right of the center line. This pulse reaches the rightear at a time 1-, later with an amplitude g Accordingly, the pulse fromadder 21 is retarded 1 seconds in delay device 24 and conformed inamplitude by amplifier 25 with a gain g to that appropriate to thelisteners left ear and supplied to the negative input of subtractor 26.Data concerning the relative magnitude of g may be obtained from thecurves on page 224 of Speech and Hearing in Communication, H. Fletcher,D. Van Nostrand Company, Incorporated, New York, New York, 1953. Thevalue of T, may be derived from experimental data for various values ofThis delayed and attenuated pulse is available at terminal 27 and, asdelivered to loudspeaker 1, provides the necessary pulse at thelisteners left ear for canceling the initial pulse radiated fromloudspeaker 2, e.g., the initial pulse of x (t) available at outputterminal 23. The correct pulse at the listeners left ear is developed bypassing the pulse from adder 21 through delay device 28 proportioned toimpart a delay 1,, to the signal and through amplifier 29 proportionedto attenuate the signal g, and delivering it to the positive input ofsubtractor 26. This component of x (t) is radiated by loudspeaker 1 andreaches the listeners ear at the correct time and with the correctmagnitude to supply the necessary directional information by which thelistener perceives a signal to originate at phantom source 3. Since thispulse also reaches the listeners right car, it is canceled by passingthe initial pulse from adder 21 through delay device 30 proportionedwith a delay 7- 4-1 and through amplifier 31 with an attenuationproportional to g,,g,,. A tandem connection of amplifiers 36 and 37,proportioned respectively with gains g, (similar to amplifier 29) and g,(similar to amplifier 25) yields the necessary product signal g g Thissignal is passed with negative polarity from subtractor 22 to terminal23 and thence to loudspeaker 2. Since this signal also reaches thelisteners left car, it must be canceled in the left channel. Similarly,all subsequent pulses must be reciprocally canceled indefinitely oruntil the magnitudes are below the threshold of perception. Since eachpulse is attenuated by a factor -g,, and delayed by T when applied tothe opposite loudspeaker for canceling purposes, the canceling impulsemust be canceled at the first loudspeaker by a pulse of amplitude (g(g,)=g, after a delay of 1,,+1-,=27-,. Thus, each pulse must be followedby a pulse attenuated by and delayed by 2T9. This is achieved by thefeedback loop portion of the network of FIG. 2. The initial pulseavailable at adder 21 is, in accordance with the invention, cycledthrough amplifier 32 with an attenuation characteristic proportional tog, and delay device 33 proportioned to have a delay time 2T9. It is thensupplied to adder 21. Accordingly, every 2T9 seconds following thedelivery of the initial impulse to adder 21, a new pulse is delivered tothe circuits previously described. The new pulse, attenuated and delayedas compared with the reference pulse, is further attenuated and delayedin the several branch circuits as described above, so that the ultimatesignals delivered to terminals 23 and 27 have the characteristicsdescribed above in Equations 11 and 12. Amplifier 32 typically includesthe tandem connections of amplifiers 34 and 35, each with gain g, (toyield the requisite gain g Other arrangements for developing a signalproportioned to g, may, of course, be used.

It will be realized that, in the previous discussion, the effects offrequency have been ignored. It has been found, however, that evenwithout frequency dependent compensation, remarkably good localizationmay be obtained with the apparatus of FIG. 2 with the several parametersproportioned as described. In particular, for angles less than about 60the phantom source is quite apparent for most listeners even withfrequency independent amplifiers. For angles greater than about 60,e.g., o 60, localization isless well defined since it is more stronglydependent on frequency. It is in accordance with the present invention,therefore, to adjust the frequency response of the amplifiers 25, 29,31, and 32 in the apparatus of FIG. 2

in accordance with the particular angle (p involved, thereby tocompensate for the fall-off in sharpness of the phan tom source atcertain (higher) frequencies.

The frequency response of the several amplifiers may be adjusted in anyconventional manner; for example, suitable filters in series with theamplifiers may be used. Alternatively, the response of the amplifiersthemselves may be suitably tailored using any of the techniques wellknown to those skilled in the art. FIG. 3 illustrates several frequencyresponse curves for the amplifiers for various angles. For an angle=22.5 for example, the required response is relatively flat for allamplifiers. For an angle =45, it has been found experimentally that theresponse of amplifiers used to adjust signals according to g, may berelatively fiat, similar to the curve g, shown for 22.5 Thecharacteristic of amplifiers used to adjust signals to 55,, however,falls oif rapidly in frequency, as shown. Similarly, for tests haveshown that the amplifiers of gain g, may retain a relatively flatresponse, and the response of those of gain g, should, for bestdirectional effects, be suitably attenuated at the higher frequencies.

For improved localization at larger angles, the delays involving T mustalso be properly adjusted. The correct delay value may be selected, forexample, from among several available delay values by means of apparatusconnected in the fashion illustrated in FIG. 4. In the figure, fixeddelay elements 41, 42, 43, 44 are tandemly connected to provide ndifierent delay values (multiples of T) at the terminals of a multiplecontact switch 45. 1- may typically be microseconds.

The localization network of FIG. 2 may thus be used as a filter toprepare suitable signals from a single one supplied to terminal 20, forenergizing a pair of spaced loudspeakers in a fashion to create thenecessary psychological information by which a listener imagines a soundto originate in a phantom source at any desired location in a plane.Although described on the basis of a single pulse, the desired spatialimpression created as the amplifier and delay elements of thelocalization network are properly adjusted, will, as mentioned above, beequally well defined in the case of any real sound source, e.g., speechor music. In general, localization will be good for signals withtransients. But, even for the extreme case of a source emitting a sinewave (in which case localization may be diflicult for both real andphantom sources depending on the frequency of the sine wave and thereverberation of the listening area), the phantom source created bysignals developed in the localization network will be as good (sharp) asfor the real source. This result is, of course, to be expected because,if a pulse is reproduced correctly at the cars, it follows from theuniqueness of Fourier transformation that all of its individualfrequency components will also be correctly reproduced.

If the network is placed between a microphone or recording transducerand a pair of loudspeakers, the recorded material may be made to appearto originate at a desired phantom location. If the recorded materialrepresents two signals of a stereophonic recording, the apparentseparation aiforded by the stereophonic recording may be considerablyenhanced by use of the localization networks in accordance with thepresent invention. The latter arrangement is illustrated in FIG. 5. Inthe figure, individual channel signals of a two-channel stereophonicrecording are supplied respectively to input terminals 51 and 52 and, ifdesired, are adjusted in gain in ordinary linear amplifiers 53 and 54.The signal developed at the output of amplifier 53 is passed throughlocalization network 55 to produce two output signals x (t) and x (t).The channel signal at the output of amplifier 54 is passed throughlocalization network 56 to develop a pair of localized signals x (t) andx '(t). Localization networks 55 and 56 are of the construction shown inFIG. 2. As described above, the paired output signals contain thenecessary clues to psychologically direct a listeners attention to aparticular phantom location. The x component signals from networks 55and 56 are combined in adder 57 and delivered to loudspeaker 1.Similarly, the x (t) signals from the networks are combined in adder 58and delivered to loudspeaker 2. By suitably selecting the gain and delayparameters of the networks 55 and 56, any desired degree of spatialspreading of the sounds emanating from loudspeakers 1 and 2 may besecured. If network 55, for example, is adjusted to have a plus 90characteristic and network 56 adjusted to have a minus 90characteristic, the stereo signals, which normally would be made toappear to originate from the two loudspeaker positions or from positionbetween them, may be made to appear to originate from points in a 180area, i.e., from anywhere in a half plane.

On occasion it is also sometimes desirable to create at the ears of thelistener sound signals, as if they actually originated at the ears,e.g., as though the listener were wearing earphones. This mode ofoperation has particular application to so-called artificial orquasistereo reproduction wherein crosstalk between the indivivdualchannels must be minimized. Typically, different versions ormodifications of a single monophonic signal are delivered independentlyto a listeners two ears to yield an effect which, .ambiophony or spatialspread is similar to true stereophonic reproduction. By suitablyadjusting the parameters of localization networks 55 and 56, the signalsradiated from loudspeakers 1 and 2 may thus be transformed as iforiginating at the listeners ears, i.e., the crosstalk component may becompletely removed.

The localization apparatus of the invention may also be used to equalizestereophonic signals before recording so that they possess the necessarypsychological directivity clues that will give rise to the phantomsource illusion when the recording is subsequently reproduced withordinary two-channel equipment. Apparatus of this sort is illustrated inFIG. 6. In the figure, a number of microphones 61-1 61-n are employed in.a studio 60 to capture sounds emanating from a variety of sources (notshown). The signals are independently amplified in amplifiers 62 anddelivered to localization networks 63. Each of the networks is adjustedfor a particular angle 5 associated with the corresponding microphone61. With the arrangement shown, therefore, the signals from themicrophones, which may be spaced close to one another in one plane, maybe made to appear to have originated from widely spaced sources. Even ifthe microphones themselves are spatially oriented, as, for example, inthe production of a stereophonic recording, the illusion of separationmay be further enhanced appreciably by passing the signals developedthrough localization networks 63. The several signals x (t) developed inthe networks 63 are combined and supplied by way of amplifier 64 tooutput terminal 65. Similarly, the x (t) signals developed at the otheroutput of each of the localization networks are combined and supplied byway of amplifier 66 to output terminal 67. Normally, the signals whichappear at terminals 65 and 67 are passed through the usual auxiliaryequipment prior to recording on disc or tape. Since the recorded signalsprepared in the manner described above contain the necessary directionalinformation, reproduction of the signals on ordinary equipment will giverise to an apparent spread of the signals over a half plane area, eventhough only two loudspeakers are used. With either of the techniquesdescribed above, the stage width produced by two loudspeakers can beconsiderably increased.

The above-described arrangements are, of course, merely illustrative ofthe application of the principles of the invention. Numerous otherarrangements may be devised by those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. A two-channel sound system characterized by a broadened stage widthcomprising, first and second loudspeakers in spaced relation,localization network means supplied with signals from a first signalsource for developing a sequence of delayed repetitions of said firstsignal, each replica having its magnitude adjusted according to a firstschedule, localization network means supplied with signals from a secondsignal source for developing a sequence of delayed repetitions of saidsecond signal, each repetition having its magnitude adjusted accordingto a second schedule, and means for supplying said repetitive signalsfrom said localization networks to said first and said secondloudspeakers, respectively, whereby the combined sound pressure fieldfrom said first and said second loudspeakers has prescribed phase andintensity characteristics at two different locations.

2. The sound system of claim 1 wherein said localization network meanscomprises: a first adding network supplied with sound signals from asource external to said network, and with periodic repetitions of saidsound signals each adjusted in magnitude by a prescribed decrement; afirst and second path for signals from said first adding means to firstand second output terminals; the first of said paths including a secondadding network, means for supplying signals from said first addingnetwork directly to said second adding network, means for supplyingsignals delayed and attenuated according to a first prescribed schedulefrom said first adding network to said second adding network, and meansfor supplying the additive output of said second adding means to saidfirst output terminal; the second of said paths including a third addingnetwork, means for supplying signals delayed and attenuated according toa second prescribed schedule from said first adding network to saidthird adding network, means for supplying signals delayed and attenuatedaccording to a third prescribed schedule from said first adding networkto said third adding network, and means for supplying the additiveoutput of said third adding means to said second output terminal.

3. A two-channel signal processing system comprising: first and secondinput terminals for receiving sound signals; first localization networkmeans for developing first and second correlated signals, each includinga plurality of variously delayed repetitions with prescribed intensityand phase characteristics, from signals supplied thereto from said firstinput terminal; second localization network means for developing firstand second correlated signals, each including a plurality of variouslydelayed repetitions with prescribed intensity and phase characteristics,from signals supplied thereto from said second input terminal; firstmeans for algebraically combining said first signals from said first andsaid second localization network means; second means for algebraicallycombining said second signals from said first and said secondlocalization network means; means for delivering said algebraiccombination of said first signals to a first output terminal; and meansfor delivering said algebraic combination of said second signals to asecond output terminal.

4. A two-channel sound system comprising; first and second sources ofsound signals, first and second loudspeakers in spaced relation; firstlocalization network means for developing first and second correlatedsignals each including a plurality of periodically delayed repetitionswith prescribed intensity and phase characteristics, from signalssupplied thereto from said first source of sound signals; secondlocalization network means for developing first and second correlatedsignals, each including a plurality of periodically delayed repetitionswith prescribed intensity and phase characteristics, from signalssupplied thereto from said second source of sound signals; first meansfor algebraically combining said first signals from said first and saidsecond localization network means; second means for algebraicallycombining said second signals from said first and said secondlocalization network means; means for delivering said algebraiccombination of said first signals to said first loudspeaker; and meansfor delivering said algebraic combination of said second signals to saidsecond loudspeaker.

5. A sound signal processing system comprising: a plurality of inputterminals for the independent reception of sound signals; a plurality oflocalization networks, each supplied with signals from one of said inputterminals, each of said localization networks including means fordeveloping first and second trains of repetitive signals whose phase,intensity and frequency characteristics are related to each other to aprescribed degree; means for individually adjusting each of saidlocalization networks to develop said first and said second correlatedsignals, respectively, according to a prescribed schedule, a differentschedule being uniquely prescribed for each one of said localizationnetworks; means for algebraically combining all of said first developedsignals, means for algebraically combining all of said second developedsignals; means for delivering said combination of first signals to afirst output terminal; and means for delivering said combination of saidsecond signals to a second output terminal.

6. Apparatus for creating arbitrarily located sound images comprising, apair of loudspeakers, means for independently energizing each of saidloudspeakers, localization network means associated with each of saidenergizing means for conforming the intensity, phase, and frequencycharacteristics of signals supplied thereto, in accordance with aprescribed schedule such that the resultant sound pressure wavesradiated by both of said loudspeakers together combine at a location inspaced relation to said pair of loudspeakers to produce a resultantsound pressure wave whose subjective indicia indicate an origin otherthan in the area immediately encompassed by said pair of loudspeakers.

7. Apparatus for creating arbitrarily located sound images outside ofthe sector defined by a pair of loudspeakers and a point in front of andon the center line between them comprising a pair of correlated soundsignals, means for independently developing from each of said signals asequence of selectively attenuated and delayed repetitions of saidsignals, means for algebraically combining said first and secondsequences according to a prescribed schedule to produce first and seconddriving signals, a pair of loudspeakers spaced apart from one another,and means for energizing each of said loudspeakers with one of saiddriving signals.

8. Apparatus for creating arbitrarily located sound images comprisingfirst and second loudspeakers spaced apart from one another, a source offirst and second correlated signals whose relative phases and amplitudesspecify a distinct spatial location of sound origin with regard to apair of fixed points, means for iteratively producing from said firstsignal a number of signal replicas spaced apart in time and individuallyadjusted in magnitude in accordance with a first prescribed schedule,means for iteratively producing from said second signal a number ofsignal replicas spaced apart in time and individually adjusted inmagnitude in accordance with a second prescribed schedule, said firstand said second schedules being selected with relation to one another inaccordance with the spatial relation of said first and said secondloudspeakers and with relation to the sound pressure wave responsedesired at said spatial location of sound origin, means for supplyingsaid first signal and its adjusted replicas to said first loudspeaker,and means for supplying said second signal and its adjusted replicas tosaid second loudspeaker.

9. In a two-channel sound system an input terminal, means for supplyingsound signals to said input terminal, means for developing from saidapplied signals a sequence of replicas thereof each delayed by aprescribed interval and each adjusted in intensity and phase inaccordance with a first prescribed schedule, a first output terminal,means for delivering said developed signals adjusted according to saidfirst schedule to said first output terminal, means for developing fromsaid applied signals a sequence of replicas thereof each delayed by aprescribed interval and each adjusted in intensity and phase inaccordance with a second prescribed schedule, a second output terminal,and means for delivering said developed signals adjusted according tosaid second schedule to said second output terminal, said first and saidsecond schedules being selected to yield signals at said first and saidsecond output terminals with a desired degree of phase and intensitycorrelation.

Burstein Stereo Amplifier controls: Electronics World, August 1959, pp.57, 122.

Burstein Amplifiers for Stereo: October 1958, pp. 40-45.

Radio-Electronics,

ROBERT H. ROSE, Primary Examiner.

1. A TWO-CHANNEL SOUND SYSTEM CHARACTERIZED BY A BROADENED STAGE WIDTHCOMPRISING, FIRST AND SECOND LOUDSPEAKERS IN SPACED RELATION,LOCALIZATION NETWORK MEANS SUPPLIED WITH SIGNALS FROM A FIRST SIGNALSOURCE FOR DEVELOPING A SEQUENCE OF DELAYED REPETITIONS OF SAID FIRSTSIGNAL, EACH REPLICA HAVING ITS MAGNITUDE ADJUSTED ACCORDING TO A FIRSTSCHEDULE, LOCALIZATION NETWORK MEANS SUPPLIED WITH SIGNALS FROM A SECONDSIGNAL SOURCE FOR DEVELOPING A SEQUENCE OF DELAYED REPETITIONS OF SAIDSECOND SIGNAL, EACH REPETITIVE HAVING ITS MAGNITUDE ADJUSTED ACCORDINGTO A SECOND SCHEDULE, AND MEANS FOR SUPPLYING SAID REPETITIVE